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Motivated by evidences favoring a rapid and late hydrogen reionization process completing at z ∼ 5.2–5.5 and mainly driven by rare and luminous sources, we have reassessed the estimate of the space density of ultra-luminous QSOs at z ∼ 5 in the framework of the QUBRICS survey. A ∼ 90% complete sample of 14 spectroscopically confirmed QSOs at M1450 ≤ −28.3 and 4.5 ≤ z ≤ 5.0 has been derived in an area of 12,400 deg2, thanks to multiwavelength selection and Gaia astrometry. The space density of z ∼ 5 QSOs within −29.3 ≤ M1450 ≤ −28.3 is three times higher than previous determinations. Our results suggest a steep bright-end slope for the QSO luminosity function at z ∼ 5 and a mild redshift evolution of the space density of ultrabright QSOs (M1450 ∼ −28.5) at 3 < z < 5.5, in agreement with the redshift evolution of the much fainter active galactic nucleus (AGN) population at M1450 ∼ −23. These findings are consistent with a pure density evolution for the AGN population at z > 3. Adopting our z ∼ 4 QSO luminosity function and applying a mild density evolution in redshift, a photoionization rate of has been obtained at z = 4.75, assuming an escape fraction of ∼70% and a steep faint-end slope of the AGN luminosity function. The derived photoionization rate is ∼50–100% of the ionizing background measured at the end of the reionization epoch, suggesting that AGNs could play an important role in the cosmological reionization process. [ABSTRACT FROM AUTHOR]

Based on results by recent surveys, the number of bright quasars at redshifts z > 3 is being constantly revised upward. The current consensus is that at bright magnitudes (M1450 ≤ −27) the number densities of such sources could have been underestimated by a factor of 30%–40%. In the framework of the QUBRICS survey, we identified 58 bright QSOs at 3.6 ≤ z ≤ 4.2, with magnitudes ipsf ≤ 18, in an area of 12400 deg2. The uniqueness of our survey is underlined by the fact that it allows us, for the first time, to extend the sampled absolute magnitude range up to M1450 = −29.5. We derived a bright-end slope of β = −4.025 and a space density at 〈M1450〉 = −28.75 of 2.61 × 10−10 Mpc−3 comoving, after taking into account the estimated incompleteness of our observations. Taking into account the results of fainter surveys, active galactic nuclei (AGNs) brighter than M1450 = −23 could produce at least half of the ionizing emissivity at z ∼ 4. Considering a mean escape fraction of 0.7 for the QSO and AGN population, combined with a mean free path of 41.3 proper Mpc at z = 3.9, we derive a photoionization rate of , produced by AGNs at M1450 < −18, that is, ∼100% of the measured ionizing background at z ∼ 4. [ABSTRACT FROM AUTHOR]

We compare the maximal abundance of massive systems predicted in different dynamical dark energy (DDE) models at high redshifts z ≈ 4–7 with the measured abundance of the most massive galaxies observed to be already in place at such redshifts. The aim is to derive constraints for the evolution of the dark energy equation-of-state parameter w that are complementary to existing probes. We adopt the standard parameterization for the DDE evolution in terms of the local value w0 and of the lookback time derivative of the equation of state wa. We derive constraints on combinations of (w0, wa) in the different DDE models by using three independent probes: (i) the observed stellar mass function of massive objects at z ≥ 6 derived from the CANDELS survey; (ii) the estimated volume density of massive halos derived from the observation of massive, star-forming galaxies detected in the submillimeter range at z ≈ 4; (iii) the rareness of the most massive system (with an estimated gas mass exceeding 3 × 1011M⊙) observed to be in place at z ≈ 7, a far-infrared–luminous object recently detected in the South Pole Telescope survey. Finally, we show that the combination of our results from the three above probes excludes a sizable fraction of the DDE parameter space wa ≳ −3/4 − (w0 + 3/2) presently allowed (or even favored) by existing probes. [ABSTRACT FROM AUTHOR]

It is widespread opinion that hydrogen reionization is mainly driven by primeval star-forming galaxies, with high-z active galactic nuclei (AGNs) having a minor role. Recent observations, however, challenge this notion, indicating a number of issues related to a galaxy-driven reionization scenario. We provide here an updated assessment of the space density of relatively faint (M1450 ∼ −22.5) AGNs at zspec ∼ 5.5 in order to improve the estimate of the photoionization rate contribution from accreting supermassive black holes. Exploiting deep UV rest-frame ground-based spectra collected at the Very Large Telescope on the CANDELS/GOODS-South field and deep Chandra X-ray images in the CANDELS/GOODS-North and EGS areas, we find two relatively bright (M1450 ∼ −22.5) AGNs at zspec ∼ 5.5. We derive an AGN space density of Φ = 1.29 × 10−6 cMpc−3 at z ∼ 5.5 and M1450 ∼ −22.5 by simply dividing their observed number by the cosmological volume in the range 5.0 < z < 6.1. Our estimate does not consider corrections for incompleteness; therefore, it represents a lower limit, although uncertainties due to cosmic variance can still be significant. This value supports a high space density of AGNs at z > 5, in contrast with previous claims mostly based on standard color selection, possibly affected by significant incompleteness. Our estimate for the AGN photoionization rate at z ∼ 5.5 is in agreement with the observed values at similar redshifts, which are needed to keep the intergalactic medium highly ionized. Upcoming James Webb Space Telescope and giant ground-based telescopes observations will improve the study of high-z AGNs and their contribution to the reionization of the universe. [ABSTRACT FROM AUTHOR]

We have evaluated the diffuse intracluster light (ICL) in the central core of the galaxy cluster CL0024+17 at z ∼ 0.4 observed with the prime focus camera (Large Binocular Camera) at the Large Binocular Telescope. The measure required an accurate removal of the galaxies' light within ∼200 kpc from the center. The residual background intensity has then been integrated in circular apertures to derive the average ICL intensity profile. The latter shows an approximate exponential decline as expected from theoretical cold dark matter models where the ICL is due to the integrated contribution of light from stars that are tidally stripped from the halo of their host galaxies due to encounters with other galaxies in the cluster cold dark matter (CDM) potential. The radial profile of the ICL over the galaxies intensity ratio (ICL fraction) is increasing with decreasing radius, but near the cluster center it starts to bend and then decreases where the overlap of the halos of the brightest cluster galaxies becomes dominant. Theoretical expectations in a simplified CDM scenario show that the ICL fraction profile can be estimated from the stripped over galaxy stellar mass ratio in the cluster. It is possible to show that the latter quantity is almost independent of the properties of the individual host galaxies but mainly depends on the average cluster properties. The predicted ICL fraction profile is thus very sensitive to the assumed CDM profile, total mass, and concentration parameter of the cluster. Adopting values very similar to those derived from the most recent lensing analysis in CL0024+17, we find a good agreement with the observed ICL fraction profile. The galaxy counts in the cluster core have then been compared with that derived from composite cluster samples in larger volumes, up to the clusters virial radius. The galaxy counts in the CL0024+17 core appear flatter and the amount of bending with respect to the average cluster galaxy counts imply a loss of total emissivity in broad agreement with the measured ICL fraction. The present analysis shows that the measure of the ICL fraction in clusters can quantitatively account for the stellar stripping activity in their cores and can be used to probe their CDM distribution and evolutionary status. [ABSTRACT FROM AUTHOR]

We present the final results from our ultra-deep spectroscopic campaign with FORS2 at the ESO Very Large Telescope (VLT) for the confirmation of z ≃ 7 "z-band dropout" candidates selected from our VLT/Hawk-I imaging survey over three independent fields. In particular, we report on two newly discovered galaxies at redshift ~6.7 in the New Technology Telescope Deep Field. Both galaxies show an Lyα emission line with rest-frame equivalent widths (EWs) of the order of 15-20Å and luminosities of (2-4) x 1042 ergs-1. We also present the results of ultra-deep observations of a sample of i-dropout galaxies, from which we set a solid upper limit on the fraction of interlopers. Out of the 20 z-dropouts observed we confirm 5 galaxies at 6.6 < z < 7.1. This is systematically below the expectations drawn on the basis of lower redshift observations: in particular, there is a significant lack of objects with intermediate Lyα EWs (between 20 and 55Å). We conclude that the observed trend for the rising fraction of Lyα emission in Lyman break galaxies from z ~ 3 to z ~ 6 is most probably reversed from z ~ 6 to z ~ 7. Explaining the observed rapid change in the Lyα emitter fraction among the dropout population with reionization requires a fast evolution of the neutral fraction of hydrogen in the universe. Assuming that the universe is completely ionized at z = 6 and adopting a set of semi-analytical models, we find that our data require a change of the neutral hydrogen fraction of the order of ΔχHI~ 0.6 in a time Az ~ 1, provided that the escape fraction does not increase dramatically over the same redshift interval. [ABSTRACT FROM AUTHOR]

Photometry in B, V (down to V ~ 26 mag) is presented for two 23' x 23' fields of the Andromeda galaxy (M31) that were observed with the blue channel camera of the Large Binocular Telescope during the Science Demonstration Time. Each field covers an area of about 5.1 x 5.1 kpc² at the distance of M31 (μM31 ~ 24.4 mag), sampling, respectively, a northeast region close to the M31 giant stream (field S2) and an eastern portion of the halo in the direction of the galaxy minor axis (field H1). The stream field spans a region that includes Andromeda's disk and giant stream, and this is reflected in the complexity of the color--magnitude diagram of the field. One corner of the halo field also includes a portion of the giant stream. Even though these demonstration time data were obtained under non-optimal observing conditions, the B photometry, which was acquired in time-series mode, allowed us to identify 274 variable stars (among which 96 are bona fide and 31 are candidate RR Lyrae stars, 71 are Cepheids, and 16 are binary systems) by applying the image subtraction technique to the selected portions of the observed fields. Differential flux light curves were obtained for the vast majority of these variables. Our sample mainly includes pulsating stars that populate the instability strip from the Classical Cepheids down to the RR Lyrae stars, thus tracing the different stellar generations in these regions of M31 down to the horizontal branch of the oldest (t ~ 10 Gyr) component. [ABSTRACT FROM AUTHOR]

We present an upper limit for the relative escape fraction Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. of ionizing radiation at z ~ 3.3 using a sample of 11 Lyman break galaxies (LBGs) with deep imaging in the U band obtained with the Large Binocular Camera, mounted on the prime focus of the Large Binocular Telescope. We selected 11 LBGs with secure redshifts in the range 3.27 < z < 3.35, from three independent fields. We stacked the images of our sources in the R and U bands, which correspond to an effective rest-frame wavelength of 1500 Å and 900 Å, respectively, obtaining a limit in the U band image of &be;30.7 mag (AB) at 1σ. We derive a 10. upper limit of Due to image rights restrictions, multiple line equation(s) cannot be graphically displayed. ~ 5%, which is one of the lowest values found in the literature so far at z ~ 3.3. Assuming that the upper limit for the escape fraction that we derived from our sample holds for all galaxies at this redshift, the hydrogen ionization rate that we obtain (Γ -12 < 0.3 s-1) is not enough to keep the intergalactic medium ionized and a substantial contribution to the UV background by faint active galactic nuclei is required. Since our sample is clearly still limited in size, larger z ~ 3 LBG samples at similar or even greater depths are necessary to confirm these results on a more firm statistical basis. [ABSTRACT FROM AUTHOR]

The study of the space density of bright active galactic nuclei (AGNs) at z > 4 has been subject to extensive effort given its importance in the estimation of cosmological ionizing emissivity and growth of supermassive black holes. In this context we have recently derived high space densities of AGNs at z ∼ 4 and −25 < M1450 < −23 in the Cosmic Evolution Survey (COSMOS) field from a spectroscopically complete sample. In the present paper we attempt to extend the knowledge of the AGN space density at fainter magnitudes (−22.5 < M1450 < −18.5) in the 4 < z < 6.1 redshift interval by means of a multiwavelength sample of galaxies in the Cosmic Assembly Near-IR Deep Extragalactic Legacy Survey (CANDELS) GOODS-South, GOODS-North, and EGS fields. We use an updated criterion to extract faint AGNs from a population of near-IR (rest-frame UV) selected galaxies at photometric z > 4 showing X-ray detection in deep Chandra images available for the three CANDELS fields. We have collected a photometric sample of 32 AGN candidates in the selected redshift interval, six of which having spectroscopic redshifts. Including our COSMOS sample as well as other bright QSO samples allows a first guess on the shape of the UV luminosity function (LF) at z ∼ 4.5. The resulting emissivity and photoionization rate appear consistent with that derived from the photoionization level of the intergalactic medium at z ∼ 4.5. An extrapolation to z ∼ 5.6 suggests an important AGN contribution to the ionization of intergalactic medium if there are no significant changes in the shape of the UV LF. [ABSTRACT FROM AUTHOR]

Recent advances in observations have provided a wealth of measurements of the expansions of outflows in galactic disks out to large radii in a variety of galactic hosts. To provide an updated baseline for the interpretation of such data, and to assess to what extent the present status of the modeling is consistent with the existing observations, we provide a compact two-dimensional description for the expansion of active galactic nucleus (AGN)-driven shocks in realistic galactic disks with exponential gas density profiles in a disk geometry. We derive solutions for the outflow expansion and the mass outflow rates in different directions with respect to the plane of the disk. These are expressed in terms of the global properties of the host galaxy and of the central AGN to allow for an easy and direct comparison with existing observations in a variety of galactic hosts with measured properties, and out to distances of ∼10 kpc from the center. The results are compared with a state-of-the-art compilation of observed outflows in 19 galaxies with different measured gas and dynamical mass, allowing for a detailed, one-by-one comparison with the model predictions. The agreement we obtain for a wide range of host galaxy gas mass () and AGN bolometric luminosity () provides a quantitative systematic test for the modeling of AGN-driven outflows in galactic disks. We also consider a larger sample of 48 objects in galaxies with no reliable measurements of the gas and dynamical mass. In this case, we perform a comparison of the model predictions for different bins of AGN luminosities assuming different reference values for the gas mass and dynamical mass derived from average scaling relations. Finally, we reconsider the AGN wind scaling laws empirically derived by many authors in light of the results from our updated models. The encouraging, quantitative agreement of the model predictions with a wide set of existing observations constitutes a baseline for the interpretation of forthcoming data, and for a more detailed treatment of AGN feedback in galaxy formation models. [ABSTRACT FROM AUTHOR]

Identifying the source population of ionizing radiation, responsible for the reionization of the universe, is currently a hotly debated subject with conflicting results. Studies of faint, high-redshift star-forming galaxies, in most cases, fail to detect enough escaping ionizing radiation to sustain the process. Recently, the capacity of bright quasi-stellar objects to ionize their surrounding medium has been confirmed also for faint active galactic nuclei (AGNs), which were found to display an escaping fraction of ∼74% at z ∼ 4. Such levels of escaping radiation could sustain the required UV background, given the number density of faint AGNs is adequate. Thus, it is mandatory to accurately measure the luminosity function of faint AGNs (L ∼ L*) in the same redshift range. For this reason we have conducted a spectroscopic survey, using the wide field spectrograph IMACS at the 6.5 m Baade Telescope, to determine the nature of our sample of faint AGN candidates in the COSMOS field. This sample was assembled using photometric redshifts, color, and X-ray information. We ended up with 16 spectroscopically confirmed AGNs at down to a magnitude of iAB = 23.0 for an area of 1.73 deg2. This leads to an AGN space density of (corrected) at z ∼ 4 for an absolute magnitude of M1450 = −23.5. This is higher than previous measurements and seems to indicate that AGNs could make a substantial contribution to the ionizing background at z ∼ 4. Assuming that AGN physical parameters remain unchanged at higher redshifts and fainter luminosities, these sources could be regarded as the main drivers of cosmic reionization. [ABSTRACT FROM AUTHOR]

We analyze a sample of z-dropout galaxies in the CANDELS GOODS South and UDS fields that have been targeted by a dedicated spectroscopic campaign aimed at detecting their Lyα line. Deep IRAC observations at 3.6 and 4.5 μm are used to determine the strength of optical emission lines affecting these bands at z ∼ 6.5–6.9 in order to (1) investigate possible physical differences between Lyα emitting and non-emitting sources; (2) constrain the escape fraction of ionizing photons; and (3) provide an estimate of the specific star formation rate at high redshifts. We find evidence of strong [O iii]+Hβ emission in the average (stacked) SEDs of galaxies both with and without Lyα emission. The blue IRAC [3.6]–[4.5] color of the stack with detected Lyα line can be converted into a rest-frame equivalent width EW([O iii]+Hβ) = Å assuming a flat intrinsic stellar continuum. This strong optical line emission enables a first estimate of on the escape fraction of ionizing photons from Lyα detected objects. The objects with no Lyα line show less extreme EW([O iii]+Hβ) = Å, suggesting different physical conditions of the H ii regions with respect to Lyα-emitting ones, or a larger . The latter case is consistent with a combined evolution of and the neutral hydrogen fraction as an explanation of the lack of bright Lyα emission at z > 6. A lower limit on the specific star formation rate, SSFR > 9.1 Gyr−1 for galaxies at these redshifts can be derived from the spectroscopically confirmed sample. [ABSTRACT FROM AUTHOR]

We have discovered 11 ultra-faint (r ≲ 22.1) low surface brightness (LSB, central surface brightness 23 ≲ μr ≲ 26) dwarf galaxy candidates in one deep Virgo field of just 576 arcmin2 obtained by the Large Binocular Camera at the Large Binocular Telescope. Their association with the Virgo cluster is supported by their distinct position in the central surface brightness—total magnitude plane with respect to the background galaxies of similar total magnitude. They have typical absolute magnitudes and scale sizes, if at the distance of Virgo, in the range −13 ≲ Mr ≲ −9 and 250 ≲ rs ≲ 850 pc, respectively. Their colors are consistent with a gradually declining star formation history with a specific star formation rate of the order of 10−11 yr−1, i.e., 10 times lower than that of main sequence star-forming galaxies. They are older than the cluster formation age and appear to be regular in morphology. They represent the faintest extremes of the population of low luminosity LSB dwarfs that has recently been detected in wider surveys of the Virgo cluster. Thanks to the depth of our observations, we are able to extend the Virgo luminosity function down to Mr ∼ −9.3 (corresponding to total masses M ∼ 107M⊙), finding an average faint-end slope α ≃ −1.4. This relatively steep slope puts interesting constraints on the nature of the dark matter and, in particular, on warm dark matter (WDM) often invoked to solve the overprediction of the dwarf number density by the standard cold dark matter scenario. We derive a lower limit on the WDM particle mass >1.5 keV. [ABSTRACT FROM AUTHOR]

We present new results from our search for z ∼ 7 galaxies from deep spectroscopic observations of candidate z dropouts in the CANDELS fields. Despite the extremely low flux limits achieved by our sensitive observations, only two galaxies have robust redshift identifications, one from its Lyα emission line at z = 6.65, the other from its Lyman break, i.e., the continuum discontinuity at the Lyα wavelength consistent with a redshift of 6.42 but with no emission line. In addition, for 23 galaxies we present deep limits in the Lyα equivalent width derived from the nondetections in ultradeep observations. Using this new data as well as previous samples, we assemble a total of 68 candidate z ∼ 7 galaxies with deep spectroscopic observations, of which 12 have a line detection. With this much enlarged sample we can place solid constraints on the declining fraction of Lyα emission in z ∼ 7 Lyman-break galaxies compared to z ∼ 6, both for bright and faint galaxies. Applying a simple analytical model, we show that the present data favor a patchy reionization process rather than a smooth one. [ABSTRACT FROM AUTHOR]

We have evaluated the contribution of the active galactic nuclei (AGN) population to the ionization history of the universe based on a semi-analytic model of galaxy formation and evolution in the cold dark matter cosmological scenario. The model connects the growth of black holes and of the ensuing AGN activity to galaxy interactions. In the model we have included a self-consistent physical description of the escape of ionizing UV photons; this is based on the blast-wave model for the AGN feedback we developed in a previous paper to explain the distribution of hydrogen column densities in AGNs of various redshifts and luminosities, due to absorption by the host galaxy gas. The model predicts UV luminosity functions for AGNs that are in good agreement with those derived from the observations especially at low and intermediate redshifts (z ∼ 3). At higher redshifts (z > 5), the model tends to overestimate the data at faint luminosities. Critical biases in both the data and in the model are discussed to explain such apparent discrepancies. The predicted hydrogen photoionization rate as a function of redshift is found to be consistent with that derived from the observations. All of the above suggests that we should reconsider the role of the AGNs as the main driver of the ionization history of the universe. [ABSTRACT FROM AUTHOR]

We use the spatially resolved, multi-band photometry in the GOODS South field acquired by the CANDELS project to constrain the nature of candidate Lyman continuum (LyC) emitters at redshift z ∼ 3.7 identified using ultradeep imaging below the Lyman limit (1σ limit of ≈30 AB in a 2″ diameter aperture). In 19 candidates out of a sample of 20 with flux detected at >3σ level, the light centroid of the candidate LyC emission is offset from that of the Lyman break galaxy (LBG) by up to 1.″5. We fit the spectral energy distribution of the LyC candidates to spectral population synthesis models to measure photometric redshifts and the stellar population parameters. We also discuss the differences in the UV colors between the LBG and the LyC candidates, and how to estimate the escape fraction of ionizing radiation (fesc) in cases, like in most of our galaxies, where the LyC emission is spatially offset from the host galaxy. In all but one case we conclude that the candidate LyC emission is most likely due to lower redshift interlopers. Based on these findings, we argue that the majority of similar measurements reported in the literature need further investigation before it can be firmly concluded that LyC emission is detected. Our only surviving LyC candidate is an LBG at z = 3.795, which shows the bluest (B – V) color among LBGs at similar redshift, a stellar mass of M ∼ 2 × 109M☼, weak interstellar absorption lines, and a flat UV spectral slope with no Lyα in emission. We estimate its fesc to be in the range 25%-100%, depending on the dust and intergalactic attenuation. [ABSTRACT FROM AUTHOR]